Communication devices such as wireless devices are also known as, e.g., User Equipments (UE), mobile terminals, terminals, wireless terminals and/or mobile stations. Terminals are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Wireless devices may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
The 3GPP Rel-13 feature “Licensed-Assisted Access” (LAA) allows LTE equipment to also operate in the unlicensed 5 GHz radio spectrum. The unlicensed 5 GHz spectrum is used as a complement to the licensed spectrum. An on-going 3GPP Rel-14 work item will add UL transmissions to LAA. Accordingly, devices connect in the licensed spectrum (primary cell or PCell) and use carrier aggregation to benefit from additional transmission capacity in the unlicensed spectrum (secondary cell or SCell). Standalone operation of LTE in unlicensed spectrum is also possible and is under development by the MuLTEfire Alliance.
Regulatory requirements, however, may not permit transmissions in the unlicensed spectrum without prior channel sensing. Since the unlicensed spectrum must be shared with other radios of similar or dissimilar wireless technologies, a so called listen-before-talk (LBT) method needs to be applied. LBT involves sensing the medium for a pre-defined minimum amount of time and backing off if the channel is busy. Today, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the IEEE 802.11 Wireless Local Area Network (WLAN) standard. This standard is known under its marketing brand “Wi-Fi”.
Long Term Evolution (LTE)
LTE uses OFDM in the downlink and DFT-spread OFDM (also referred to as single-carrier FDMA) in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. The uplink subframe has the same subcarrier spacing as the downlink and the same number of SC-FDMA symbols in the time domain as OFDM symbols in the downlink.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms as shown in FIG. 1. Each subframe comprises two slots of duration 0.5 ms each, and the slot numbering within a frame ranges from 0 to 19. For normal cyclic prefix, one subframe consists of 14 OFDM symbols. The duration of each symbol is approximately 71.4 μs.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information about which terminals data is transmitted to and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of e.g. the control information. A downlink system with CFI=3 OFDM symbols as control is illustrated in FIG. 2. The reference symbols shown there are the cell specific reference symbols (CRS) and are used to support multiple functions including fine time and frequency synchronization and channel estimation for certain transmission modes.
Uplink transmissions are dynamically scheduled, i.e., in each downlink subframe the base station transmits control information about which terminals should transmit data to the eNB in subsequent subframes, and upon which resource blocks the data is transmitted. The uplink resource grid is comprised of data and uplink control information in the PUSCH, uplink control information in the PUCCH, and various reference signals such as demodulation reference signals (DMRS) and sounding reference signals (SRS). DMRS are used for coherent demodulation of PUSCH and PUCCH data, whereas SRS is not associated with any data or control information but is generally used to estimate the uplink channel quality for purposes of frequency-selective scheduling. An example uplink subframe is shown in FIG. 4. Note that UL DMRS and SRS are time-multiplexed into the UL subframe, and SRS are always transmitted in the last symbol of a normal UL subframe. The PUSCH DMRS is transmitted once every slot for subframes with normal cyclic prefix, and is located in the fourth and eleventh SC-FDMA symbols.
From LTE Rel-11 onwards, DL or UL resource assignments can also be scheduled on the enhanced Physical Downlink Control Channel (EPDCCH). For Rel-8 to Rel-10 only the Physical Downlink Control Channel (PDCCH) is available. Resource grants are UE specific and are indicated by scrambling the DCI Cyclic Redundancy Check (CRC) with the UE-specific C-RNTI identifier.
eIMTA Signaling
In Rel-12 LTE, dynamic TDD or eIMTA (enhanced Interference Mitigation and Traffic Adaptation) was introduced as a feature. Here, the serving cell can dynamically change the cell TDD configuration on a per-frame basis. The configuration is signaled using the (e)PDCCH after scrambling with the eIMTA RNTI. The choice of the cell DL/UL subframe configuration is limited to the 7 TDD configurations in legacy LTE, therefore arbitrary DL/UL allocations within a frame are not allowed.
Carrier Aggregation
The LTE Rel-10 standard supports bandwidths larger than 20 MHz. One important requirement on LTE Rel-10 is to assure backward compatibility with LTE Rel-8. This should also include spectrum compatibility. That would imply that an LTE Rel-10 carrier, wider than 20 MHz, should appear as a number of LTE carriers to an LTE Rel-8 terminal. Each such carrier can be referred to as a Component Carrier (CC). In particular for early LTE Rel-10 deployments it can be expected that there will be a smaller number of LTE Rel-10-capable terminals compared to many LTE legacy terminals. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e. that it is possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE Rel-10 carrier. The straightforward way to obtain this would be by means of Carrier Aggregation (CA). CA implies that an LTE Rel-10 terminal can receive multiple CC, where the CC have, or at least the possibility to have, the same structure as a Rel-8 carrier. CA is illustrated in FIG. 5. A CA-capable UE is assigned a primary cell (PCell) which is always activated, and one or more secondary cells (SCells) which may be activated or deactivated dynamically.
The number of aggregated CC as well as the bandwidth of the individual CC may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same whereas an asymmetric configuration refers to the case that the number of CCs is different. It is important to note that the number of CCs configured in a cell may be different from the number of CCs seen by a terminal: A terminal may for example support more downlink CCs than uplink CCs, even though the cell is configured with the same number of uplink and downlink CCs.
Wireless Local Area Network
In typical deployments of WLAN, carrier sense multiple access with collision avoidance (CSMA/CA) is used for medium access. This means that the channel is sensed to perform a clear channel assessment (CCA), and a transmission is initiated only if the channel is declared as Idle. In case the channel is declared as Busy, the transmission is essentially deferred until the channel is deemed to be Idle.
A general illustration of the listen before talk (LBT) mechanism of Wi-Fi is shown in FIG. 6. After a Wi-Fi station A transmits a data frame to a station B, station B shall transmit the ACK frame back to station A with a delay of 16 μs. Such an ACK frame is transmitted by station B without performing a LBT operation. To prevent another station interfering with such an ACK frame transmission, a station shall defer for a duration of 34 μs (referred to as DIFS) after the channel is observed to be occupied before assessing again whether the channel is occupied. Therefore, a station that wishes to transmit first performs a CCA by sensing the medium for a fixed duration DIFS. If the medium is idle then the station assumes that it may take ownership of the medium and begin a frame exchange sequence. If the medium is busy, the station waits for the medium to go idle, defers for DIFS, and waits for a further random backoff period.
In the above basic protocol, when the medium becomes available, multiple Wi-Fi stations may be ready to transmit, which can result in collision. To reduce collisions, stations intending to transmit select a random backoff counter and defer for that number of slot channel idle times. The random backoff counter is selected as a random integer drawn from a uniform distribution over the interval of [0, CW]. The default size of the random backoff contention window, CWmin, is set in the IEEE specs. Note that collisions can still happen even under this random backoff protocol when there are many stations contending for the channel access. Hence, to avoid recurring collisions, the backoff contention window size CW is doubled whenever the station detects a collision of its transmission up to a limit, CWmax, also set in the IEEE specs. When a station succeeds in a transmission without collision, it resets its random backoff contention window size back to the default value CWmin.
LTE in Unlicensed Spectrum
Up to now, the spectrum used by LTE is dedicated to LTE. This has the advantage that LTE system does not need to care about the coexistence issue and the spectrum efficiency can be maximized. However, the spectrum allocated to LTE is limited which cannot meet the ever increasing demand for larger throughput from applications/services. Therefore, Rel-13 LAA extended LTE to exploit unlicensed spectrum in addition to licensed spectrum. Unlicensed spectrum can, by definition, be simultaneously used by multiple different technologies. Therefore, LTE needs to consider the coexistence issue with other systems such as IEEE 802.11 (Wi-Fi). Operating LTE in the same manner in unlicensed spectrum as in licensed spectrum can seriously degrade the performance of Wi-Fi as Wi-Fi will not transmit once it detects the channel is occupied.
Furthermore, one way to utilize the unlicensed spectrum reliably is to transmit essential control signals and channels on a licensed carrier. That is, as shown in FIG. 7, a UE is connected to a PCell in the licensed band and one or more SCells in the unlicensed band. In this application we denote a secondary cell in unlicensed spectrum as licensed-assisted access secondary cell (LAA SCell). In the case of standalone operation as in MuLTEfire, no licensed cell is available for uplink control signal transmissions.
The maximum channel occupancy time (MCOT) of a single DL+UL TXOP in unlicensed bands is limited by regional regulatory restrictions. For example, in Europe, EN BRAN is considering the following MCOT rules:                Specify max TxOP=6 ms available for 100% of the time        Specify max TxOP=8 ms is available for 100% of time with a minimum pause of [TBD] μs (in order of 100's μs) after a maximum transmission of 6 ms        Specify max TxOP=10 ms is available for [TBD3] % of the time.DL Signaling in C-PDCCH in LAA        
In Rel-13 DL-only LAA, it was agreed that the length of an ending partial DL subframe, where the partial subframe is the last subframe of a DL burst, can be indicated to UEs via common PDCCH (C-PDCCH) signaling. If the UE detects common PDCCH DCI referring to subframe n in subframes n−1 or n, the UE may assume the number of OFDM symbols in subframe n according to the detected DCI. If the end subframe is a partial DL subframe, then the end partial subframe configuration of a DL transmission burst is indicated to the UE in the end subframe and the previous subframe. The following information can be signaled in Rel-13 LAA (e)PDCCH using four bits in DCI Format 1C (as seen by Table 1):
TABLE 1C-PDCCH signaling for ending partialDL subframes in Rel-13 LAA0Next subframe is 3 OFDM symbols1Next subframe is 6 OFDM symbols2Next subframe is 9 OFDM symbols3Next subframe is 10 OFDM symbols4Next subframe is 11 OFDM symbols5Next subframe is 12 OFDM symbols6Next subframe is full (14 Symbols)7Current subframe is partial 3 OFDM symbols8Current subframe is partial 6 OFDM symbols9Current subframe is partial 9 OFDM symbols10Current subframe is partial 10 OFDM symbols11Current subframe is partial 11 OFDM symbols12Current subframe is partial 12 OFDM symbols13Current subframe is full (14 Symbols) and end of transmission14Reserved15ReservedUCCH and Cross-TXOP Scheduling in MuLTEfire
Two forms of PUCCH transmission have been defined for MuLTEfire: a short PUCCH (sPUCCH) comprising between two to six symbols in time, and a longer, enhanced PUCCH (ePUCCH) which spans one subframe in time, as shown in FIG. 8. The sPUCCH occurs immediately after the DwPTS portion of a partial DL subframe as indicated by C-PDCCH signaling from Rel-13 LAA, while the ePUCCH can be multiplexed with PUSCH transmissions in 1-ms UL subframes. For the triggering of ePUCCH transmissions, both common PDCCH (C-PDCCH) or UL grant (DCI based) based triggers are supported, eNB can use either or both mechanisms.
Regarding UL scheduling in MuLTEfire, the UL grant and the corresponding UL transmission(s) do not need to be contained within the same TXOP. The following procedures are supported when scheduling UL transmissions across TXOPs:
Option 1: eNB schedules UE with a fixed time relationship between grants and transmission, where the delay between grant and start of UL transmission can be 4 ms or longer. The eNB may signal the type of LBT to be performed by the UE in the grant (e.g., 25 μs or Cat-4 LBT depending on transmission time relative to TxOP limit).
Option 2: eNB schedules UE without a fixed time relationship between grant and UL transmission. UE transmits after a minimum delay m1 since the grant was received, m1>=4 ms, and only after further receiving a trigger sent by eNB on the C-PDCCH. The UL LBT is 25 μs in this case. If the C-PDCCH trigger is not received within a time m2, the UE does not transmit and drops the grant. The parameter m1 is carried in the UL grant, while parameter m2 is configured using higher-layer signalling.